1. Field of the Invention
The present invention relates to a spark plug used in an internal combustion engine.
2. Description of the Related Art
Conventionally, a spark plug for an internal combustion engine such as an automobile engine employs a Pt (platinum) alloy chip welded to an end of an electrode for use as a spark discharge portion having improved spark consumption resistance. However, due to high cost and a relatively low melting point of 1769.degree. C., platinum is not satisfactory as a spark-consumption-resistant material for spark plug use. Thus, there has been proposed use of Ir (iridium), which is inexpensive and has a higher melting point of 2454.degree. C., as a material for a chip.
However, since Ir tends to produce a volatile oxide and be consumed at a high temperature zone ranging from 900.degree. C. to 1000.degree. C., a spark discharge portion formed from Ir involves a problem of consumption stemming from oxidation/volatilization rather than spark consumption. Accordingly, an Ir chip shows good endurance under low temperature conditions as in city driving, but has a problem of a significant reduction in endurance in highway driving. Thus, an attempt has been made to suppress consumption of a chip stemming from oxidation/volatilization of Ir, by adding an appropriate element to an alloy used as a material for a chip. For example, Japanese Patent Application Laid-Open (kokai) No. 9-7733 discloses a spark plug whose chip is improved in high-temperature heat resistance and consumption resistance by suppression of oxidation/volatilization of Ir through addition of Rh (rhodium). Also, in order to suppress oxidation/volatilization of Ir, there has been proposed use as a constituent material of a spark discharge portion, a material obtained through dispersion of a rare-earth oxide such as Y.sub.2 O.sub.3 into Ir (see Japanese Patent Application Laid-Open (kokai) No. 7-37677). However, in recent years, the temperature range in which spark plugs are used has become higher with a recent increase in engine output, and therefore, a spark plug having more excellent durability is demanded.